1. Field of the Invention
The present invention relates to fouling resistant coverings and more specifically to novel elastomeric materials and methods for incorporating biocidal agents therein which are capable of surviving for extended periods of time in an aqueous environment.
2. Description of the Prior Art
Marine organisms which attach themselves to submerged surfaces of ships cause increased resistance to water passage and such friction effects an operational speed reduction and contributes to increased fuel consumption. By adhering either on metal or rubber substrates, such organism enhance the ship's signature by providing a source of additional background noise. Encrustation also adversely affects equipment functions, particularly when they attach themselves to such acoustical devices as sonar domes and sonar shields. In the case of sonar domes, which are often provided with smooth rubber surfaces, these marine organisms not only destroy the surface integrity of the domes but also provide sites which tend to deflect and scatter the sonar beams. The accumulation of various marine organisms on bouys, pier pilings, ships hulls and the like and in high cavitation and erosion situations such as on the blades of propellers, legs of hydrofoils, and on discharge pumps, is deleterious to reliability and greatly increses equipment maintenance.
To prevent the accumulation of marine organisms on rubber and metal substrates, the prior art illustrates a number of combinations of polymer materials that contain biocidal agents to resist fouling on underwater surfaces. These are exemplified by, for example, U.S. Pat. No. 3,426,473. This antifoulant covering uses neoprene rubber impregnated with an organometal toxicant such as bis (tri-n-butyltin) oxide as a reservoir layer to which is laminated an elastomer sheet which serves as a toxic transfer control sheet This material is produced as a proprietary antifoulant neoprene sheet marketed under the tradename of NO-FOUL (a registered trademark of the B.F. Goodrich Co.). The major drawbacks of this material are its high application cost and the rigid quality control that is required during bonding of the cured sheets to ensure proper surface adhesion. Also, NO-FOUL evidences poor impact resistance and there exists the possibility of worker exposure to airborne organotin during application.
As a consequence, several improved covering materials have been developed. One example, as shown in U.S. Pat. No. 3,497,990, describes a cellular antifouling covering for submerged marine objects. The covering consists of a layer of foam material having interconnected cells containing diffusible antifoulant material. The antifoulant material diffuses through the permeable elastomeric cover layer to replace any material which has been washed away by the sea water. Another example, in U.S. Pat. No. 3,505,758, illustrates an antifouling covering for submerged marine objects, such as, sonar domes. This covering consists of a double-wall, rubber coated fabric reservoir which carries an intermediate layer of diffusable toxic or repellant material. A modular antifouling tile is disclosed in U.S. Pat. No. 4,401,703. This system employs a plurality of discrete tile units which are individually formed with internal reservoirs for containing antifoulant toxicants that diffuse through the elastomeric matrix of the covering surface.
However, all of the above coverings are based upon a leachable reservoir principle which permits insitu replenishment of the toxicant. The main disadvantage of these arrangements remains the high initial rate at which the antifoulants diffuse into the immersion waters. All exhibit toxicant release rates which decrease logarithmically with time which is characteristic of diffusion-controlled release coatings. That is, considerable amounts of toxicant is released intially which decreases, within a relatively short period of time, typically 14-18 months, to a residual toxic release rate that is insufficent to control marine growth yet remains in the covering even though no longer effective.
Several U.S. patents which are currently assigned to the United States Government are also pertinent to the present invention. U.S. Pat. No. 3,979,354 describes an antifouling composition comprising an organotin polymer wherein the tin is chemically combined in the polymer. The polymer bound tin is effective in reducing the leaching rate of the organometal from the compound. U.S. Pat. Nos. 4,075,319 and 4,082,709 bond organotin to polyester resins and vinyl polymer backbones such as the homopolymers and copolymers of acrylic and methacrylic acid monomers or copolymers of methyl vinyl ether and maleic acid. These resins exhibit excellent antifouling performance when exposed as neat resins and perform well as base resins in hull coatings which, when properly formulated, are suitable for use as film forming coatings such as antifouling paints.
The many attendant advantages of antifoulant paints based upon organotin acrylate polymers, including a constant delivery rate of the antifoulant, has caused many manufacturers to develop organotin polymer coating systems. Since a lixiviation/hydrolysis process of the paint film is relied upon to deliver the toxicant, some formulations have concentrated upon varying the polymer to control the erosion rate, while others, as evidenced for example by U.S. Pat. No. 4,021,392, have included hydrophobic additives such as chlorinated waxes, chlorinated rubbers, polyacrylate esters, and silicones as retarders.
Thus, while organometal polymers are known to be used in the field of paint technology, previous attempts to incorporate significant antifouling protection by compounding these toxic containing compositions into an elastomeric material have resulted in either complete degradation of the physical properties or an unacceptably short service life. Materials such as NO-FOUL which use free organometals physically mixed with the neoprene rubber are not reacted solutions and thus represent only mechanical suspensions with the result that these types of materials may contain nonuniform dispersions.